(1) Field of the Invention
This invention relates to a semiconductor light-emitting element, particularly usable for a white light-emitting diode.
(2) Related Art Statement
Recently, various light-emitting diodes (LEDs) are widely available. LEDs are expected for illuminating use as well as displaying use because of their low electric power consumption, long life time, CO2 gas reduction originated from the reduction of the high energy consumption such as the low electric power consumption, and thus, much demand for the LEDs are expected.
As of now, the LEDs are made of various semi-conducting materials such as GaAs-based semi-conducting materials, AlGaAs-based semi-conducting materials, GaP-based semi-conducting materials, GaAsP-based semi-conducting materials and InGaAlP-based semi-conducting materials, and thus, can emit various color lights from red to yellow-green. Therefore, the LEDs are employed particularly for various displaying uses. Recently, blue and green LEDs have been realized by using GaN-based semi-conducting materials. As a result, selecting a given LED, a given color light from red to blue, that is, within visible light range, can be obtained from the LED, and full-color displaying is also realized. Moreover, white light-emitting diodes (white LEDs) are being realized by using RGB LED chips or using two color light-emitting diodes composed of blue LEDs with a yellow fluorescent substance thereon. As a result, LED illumination is being realized at present.
However, the white LED using the RGB LED chips requires higher cost because the plural LED chips are employed, so that in view of the cost, it is difficult to employ the white LED for illumination use. On the other hand, full color can not be recognized by the white LED using the two color light-emitting diode because it employs only two primary colors, not three primary colors. Moreover, in the white LED, the brightness of only about 25 lm/w can be realized, which is very small as compared with the brightness of 90 lm/W of a fluorescent tube.
Therefore, a white LED employing three primary colors is strongly desired all over the world because of the low energy consumption taking environmental problems into consideration. In reality, such a white LED is intensely developed by Japanese national professions and foreign major electric-manufacturing enterprises.
Such an attempt is made as to fabricate a white LED using three or more primary colors as illuminating a three primary colors-fluorescent substance by an ultraviolet LED. This attempt is fundamentally based on the same principle as a fluorescent tube, and employs the ultraviolet LED as the ultraviolet beam from the mercury discharge in the fluorescent tube. In this case, the cost of the white LED is increased because the three primary colors-fluorescent substance is additionally employed for the ultraviolet LED. Using a GaN-based semi-conducting material, a blue LED can be realized, and then, using the GaN-based semi-conducting material, the ultraviolet LED can be realized. However, the luminous efficiency of the resulting ultraviolet LED is largely reduced, as compared with the blue LED.
The luminescence reduction is considered as follows. If the GaN-based semiconductor film is epitaxially grown on a substrate made of e.g., a sapphire single crystal, much misfit dislocations are created at the boundary between the film and the substrate due to the difference in lattice constant between the film and the substrate. The misfit dislocations are propagated in the film and a light-emitting layer provided on the film, and thus, many dislocations are created in the resulting LED.
In a blue LED or a green LED made of GaN-based semi-conducting materials, the light-emitting layer is made of an InGaN semi-conducting material. In this case, the In elements are partially located, and thus, some carriers are located and confined. Therefore, the carriers are recombined before they are moved and seized at the dislocations, so that the LED can exhibit its sufficient luminous efficiency.
That is, even though many dislocations are created in the light-emitting layer, the carriers are recombined and thus, a given luminescence is generated before they are moved and seized at the dislocation as non-luminescence centers, so that the blue LED or the green LED using the GaN-based semi-conducting materials can exhibit their high luminous efficiency.
For fabricating an ultraviolet LED, the In ratio of the light-emitting layer must be reduced. Therefore, the In elements are not almost located, and thus, the diffusion length of a carrier is elongated. As a result, the carriers are easily moved at and recombined with the dislocations in the light-emitting layer. In this way, the luminous efficiency of the ultraviolet LED is reduced due to the large amount of dislocations in the light-emitting layer, as compared with the blue LED. In this point of view, various dislocation-reducing methods have been researched and developed.
For example, such an ELO technique is proposed as fabricating a strip mask made of SiO2 during an epitaxial process and preventing the propagation of the misfit dislocations created at the boundary between the epitaxial film and a substrate. According to the ELO technique, a light-emitting layer having fewer dislocations can be formed above the substrate via strip mask. However, the ELO technique is a complicated means, so that the manufacturing cost is increased. Then, in the ELO technique, a thicker layer made of e.g., a GaN-based semi-conducting material is formed on the substrate, which results in being curved. Practically, in a device manufacturing process, when epitaxial films are formed on their respective substrates by the ELO technique, the better half of the substrates is broken. Therefore, it is difficult to employ the ELO technique in a practical device manufacturing process, particularly for LEDs.
In addition, an attempt is made to epitaxially grow a bulky GaN single crystal for reducing the dislocation density of the resulting device, for example by using a high pressure solution growth method, a vapor phase epitaxial growth method or a flux method. As of now, however, such a single crystal bulky enough to be applied for the device manufacturing process is not grown and prospected.
For fabricating a bulky GaN single crystal of low dislocation density, an attempt is made to grow a thicker GaN single crystal on a substrate made of an oxide to match in lattice the GaN single crystal by a HVPE method, and thereafter, remove the substrate, to obtain only the GaN single crystal to be used as a substrate. However, a GaN single crystal bulky enough to be industrially applied for LEDs has not yet been fabricated.
As a result, the high luminous efficiency in such a white LED as employing three or more primary colors through the illumination of a fluorescent substance by an ultraviolet LED is not technically prospected.